1. Field of the Invention
The invention relates to a fuel injection control apparatus that controls the manner of fuel injection from a fuel injection provided in an engine that executes a multiple injection in which the fuel injection from the fuel injection valve in one cycle of combustion is carried out by a plurality of divided injections.
2. Description of the Related Art
Diesel engines often employ a so-called multiple injection process in which the fuel injection from a fuel injection valve in one combustion cycle is carried out by a plurality of divided injections, in order to reduce the combustion sound and achieve a clean emission. Concrete examples of the multiple injection include an injection process in which a pre-injection of a small amount of fuel is performed prior to a main injection, an injection process in which a post-injection of a small amount of fuel is performed after a main injection, etc.
In order to properly realize the effects of execution of the multiple injection, it is important to adjust the intervals between injections (injection intervals) highly accurately to a value that is appropriate for the actual operation state of the engine. The adjustment of the injection interval is usually performed by controlling the actuation of the fuel injection valve so as to reach an optimal value that is expected on the basis of the state of operation of the engine (e.g., the engine rotation speed and the intake air amount) at every given moment.
It is to be noted that if the fuel injection from the fuel injection valve is executed, this causes occurrence of pressure pulsation within a fuel supply system for supplying fuel to the fuel injection valve. In the case where the multiple injection is executed, the execution period of the fuel injection from the fuel injection valve is very short, in comparison with the case where the multiple injection is not executed. Therefore, if pressure pulsation occurs in association with execution of a preceding fuel injection in the multiple injection process, execution of the succeeding fuel injection sometimes starts before the pressure pulsation subsides. In such a case, while the fuel pressure is still fluctuating, the succeeding fuel injection is executed, incurring the risk of causing an unnecessary change of the manner of operation of the fuel injection valve. If this unnecessary change of the manner of operation results in a change in the injection interval, the effects of execution of the multiple injection may not be properly realized.
According to the related art, for example, as in International Publication No. 2003/069146, it has been proposed that in order to restrain the adverse effect of the foregoing pressure pulsation, a correction term be calculated on the basis of the state of operation of the engine (concretely, the engine rotation speed) at every given moment, and the execution period of the succeeding injection be corrected by the correction term. According to this related-art apparatus, although the manner of occurrence of pressure pulsation varies in different states of operation of the engine, the amount of change caused in operation characteristics of the fuel injection (concretely, the valve opening timing or the valve open duration thereof) by the pressure pulsation is restrained by a control based on an optimal value that is expected from the state of operation of the engine (a value suitable to a standard engine), that is, a so-called expectation-based control.
By the way, the pressure pulsation that occurs due to execution of a preceding injection in the multiple injection varies according to the state of operation of the engine, and also varies depending on the initial individual product differences and the time-dependent changes in the fuel supply system that includes the fuel injection valve. Then, the amount of change in the operation characteristics of the fuel injection valve depending on the initial individual product differences and the time-dependent changes in the fuel supply system cannot be restrained by executing the foregoing expectation-based control of the related-art apparatus, that is, a control that is appropriate for standard engines. Therefore, even if the foregoing related-art apparatus is employed, it is not possible to avoid incurring an unnecessary change in the operation characteristics of the fuel injection valve due to occurrence of the pressure pulsation caused by execution of the preceding injection in the multiple injection process, and therefore it is not possible to avoid the decline in the accuracy in adjusting the injection interval which is caused by the unnecessary change in the operation characteristics of the fuel injection valve.